The anti-stick coatings of the present invention can be applied to any metal surface to prevent sticking during an annealing operation or to limit oxidation during an annealing operation.
A primary use for the anti-stick coatings of the present invention is their application to cold rolled, non-oriented, semi-processed silicon steels of which there are a number of well known ASTM standard grades. While not intended to be so limited, the anti-stick coatings will be described with respect to this use for purposes of an exemplary embodiment.
The term "non-oriented, semi-processed electrical steels", as used herein and in the claims, is intended to refer to those electrical steels known in the art as "semi-processed" since they have not been processed at the mill to fully develop magnetic properties. The customer must complete the processing by proper annealing. This necessary annealing (generally known in the art as a "quality anneal") involves grain growth and decarburization (depending upon the amount of decarburization accomplished in the mill), both of which are essential to development of optimum magnetic properties.
Such steels include cold rolled, non-oriented, semi-processed silicon steels; cold rolled, semi-processed carbon steels for motor laminations and the like; and semi-processed, low-oxygen silicon bearing lamination steels of the type taught in U.S. Pat. No. 3,867,211.
In the usual practice, the customer forms laminations for motors, transformers or the like from this cold rolled, non-oriented, semi-processed silicon steel. As indicated above, it is essential that these laminations be subjected to a quality anneal to develop optimum magnetic properties of the laminations. The quality anneal is usually conducted in a decarburizing atmosphere containing water vapor, such as hydrogen, hydrogen-nitrogen, or an atmosphere formed by partial combustion of gas. The quality anneal is usually conducted at a temperature within the range of from about 1400.degree. F. (760.degree. C.) to 1600.degree. F. (870.degree. C.) Temperatures at the upper end of this range tend to produce somewhat improved magnetic properties.
A major problem encountered during a quality anneal is that of adhesion or sticking of the laminations. Prior art workers have developed a number of different coatings which can be applied to the steel at the plant and which tend to prevent sticking of the laminations during the quality anneal.
For example, an electrolytic magnesium hydroxide coating was developed to prevent lamination sticking during a quality anneal at a temperature of from about 1400.degree. F. (760.degree. C.) to about 1600.degree. F. (870.degree. C.). Such a coating, however, was characterized by a number of problems. It was expensive to use and excess magnesia remained on the laminations after the quality anneal. This created dust and handling problems. The excess magnesia also got into the lubricant used during punching. If allowed to accumulate, this excess magnesia caused die wear problems. Finally, the magnetic quality of laminations provided with this coating was impaired.
Another exemplary anti-stick coating comprised a coating solution of 75% phosphoric acid diluted 8 to 1 with water and dried in a furnace at from about 1000.degree. F. (538.degree. C.) to about 1100.degree. F. (593.degree. C.), actual strip temperature of from about 700.degree. F. (371.degree. C.) to about 800.degree. F. (427.degree. C.). It was only necessary for the coating to achieve a temperature of from about 700.degree. F. (371.degree. C.) to about 800.degree. F. (427.degree. C.). Soaking for any length of time at temperature was not required.
This exemplary phosphoric acid anti-stick coating was used at quality anneal temperatures of from about 1450.degree. F. (788.degree. C.) to about 1550.degree. F. (843.degree. C.). The present day typical quality anneal temperatures are from about 1200.degree. F. (649.degree. C.) to about 1600.degree. F. (870.degree. C.). At temperatures above about 1500.degree. F. (816.degree. C.) this phosphoric acid anti-stick coating begins to decompose and cannot protect the surface of the steel from oxidation. For this reason, lamination sticking becomes a problem.
The present invention is based on the discovery that greatly improved anti-stick coatings can be formed from coating solutions taught in U.S. Pat. Nos. 3,948,786 and 3,996,073 for use in producing insulative coatings for electrical steels, when these coating solutions are so diluted as to provide a uniform coating as thin as possible to prevent lamination sticking and having a coating weight of less than 2 grams per square meter on each side. The coating solutions are applied to the cold rolled, non-oriented semi-processed silicon steel at the mill by any appropriate means and are cured by an appropriate heat treatment.
The resulting anti-stick coating constitutes a thin, hard coating which will not decompose during a quality anneal conducted at a temperature of up to at least 1650.degree. F. (900.degree. C.). Because the improved anti-stick coatings do not decompose during a quality anneal conducted at the higher temperatures and prevent lamination sticking, a number of additional benefits are obtained. For example, the improved anti-stick coatings act as nitrogen and oxygen diffusion barriers, preventing nitrogen pick-up and surface oxidation from occurring during the quality anneal. The reduction in surface oxidation not only produces laminations with improved physical appearance after the quality anneal, but also results in laminations having improved high induction magnetic properties. The anti-stick coatings of the present invention offer some rust protection to the laminations both prior to and after the quality anneal. The anti-stick coatings do not prevent decarburization from occurring during the quality anneal. By way of additional advantages, the anti-stick coatings of the present invention, since they prevent surface oxidation, enable the use of faster heating rates and/or higher temperatures. This results in increased productivity, or improved magnetic quality, or both. Further, the coatings can be punched without causing excessive die wear or chipping, are easily welded, and enable the annealed laminations to be handled by automatic stacking machines.